A passive optical network (PON) is a point-to-multipoint optical network that provides significantly greater bandwidth in an access network compared to traditional copper-based networks. Generally, a PON includes only passive components, i.e., components that do not require electrical power for operation. Repeaters, relays, processors and memory modules are typically absent. A PON generally includes an optical line termination (OLT) located in a central office (CO) or local exchange. The OLT is coupled to optical network units (ONUs), or subscriber terminals, in the field through an optical fiber. An ONU provides any necessary optical to electrical (O-E) and electrical to optical (E-O) conversion between the fiber and the copper wires that reach homes and offices in a fiber to the neighborhood (FTTN) or fiber to the curb (FTTC) implementation. In other instances, the OLT is coupled directly to an optical network terminal (ONT) such as in a fiber to the premises (FTTP) or fiber to the home (FTTH) implementation.
The OLT transmits an optical signal at a single wavelength “downstream” over an optical fiber between the OLT and the ONU (or OLTs). The ONU (or ONT) transmits an optical signal in the reverse direction, that is, “upstream” through the optical fiber at a different wavelength than the downstream optical signal. For OLTs, the downstream and upstream optical signals transmitted through the optical fiber are time-division multiplexed signals that include the individual signals for all the end users.
With an increasing consumer demand for bandwidth, wavelength division multiplexing (“WDM”) PONs have gained acceptance in the telecommunications industry. WDM PONs support substantially greater bandwidth than conventional PONs and offer the additional benefits of network security and upgradeability. Unlike standard PONs where the bandwidth available at a single wavelength is shared amongst all end users, a WDM PON system employs multiple optical transmitters where each optical transmitter transmits on a different wavelength.
Initially, the device costs for critical components of WDM PONs slowed their integration into telecommunications networks. In particular, the cost of wavelength-specific optical transmitters presented an obstacle to widespread implementation of WDM PONs. More recently WDM PONs using less expensive multi-wavelength (i.e., “colorless”) optical transmitters such as those described in Kim, Hyun Deok et al., “A Low-Cost WDM Source with an ASE Injected Fabry-Perot Semiconductor Laser,” IEEE Photonics Technology Letters, Vol. 12, No. 8, August 2000 and in Shin, Dong Jae et al., “Low-Cost WDM-PON with Colorless Bidirectional Transceivers,” Journal of Lightwave Technology, Vol. 24, No. 1, January 2006 have been used. This type of WDM PON employs a broadband seeding source that is spectrally-sliced. The light in each spectral slice is used to seed a respective one of the optical transmitter lasers to lock it to a unique WDM wavelength.
The efficiency of the colorless optical transmitters can limit the overall system performance. In particular, if the wavelength of the cavity mode of a transmitter laser is not properly aligned with respect to the wavelength corresponding to the peak optical power in the spectral slice used to seed the laser, the optical power in other cavity modes is increased with a corresponding decrease in the optical power of the primary transmitted mode. Environmental effects such as a change in temperature can cause or change a difference between the wavelength of the transmitted cavity mode and the peak of the spectral slice. In some instances the spectral slice may injection lock two cavity modes of the laser or result in mode hopping which can result in instability of the optical power output. If the spectrum of the optical signal generated by the transmitter laser is broadened due to significant optical power shifted to other cavity modes, the optical signal may be significantly attenuated by an arrayed waveguide grating (AWG) or other WDM multiplexing device.
The present invention addresses the problems set forth above and provides a convenient and cost-effective solution.